1. Field of the Invention
The present invention relates to the field of liquid depth filtration and, more particularly, to dry filter sheets for the filtration of very small particles.
2. Description of the Related Art
Many liquids are filtered to achieve clarity and purity of the final product. For example, water and such products as wine, beer, distilled spirits, fruit juices, pharmaceuticals, cosmetics, etc., are desirably filtered to remove very small particulates which cause clouding or sediment and which might go back into solution. If allowed to remain in the liquid, particulate matter which is biological in nature such as yeast, bacteria, and viruses, may change the liquid into an undesirable product by continued fermentation or other processes, thus reducing shelf life.
Liquid depth filtration has long been used to remove solids by forcing the liquid through a porous medium. Early filtering media included sand beds, cloth, and paper. Early in this century it was found that filter sheets made from asbestos and cellulose pulp were excellent filter media. Asbestos, which is a naturally occurring mineral, could be fibrillated to very small diameters and blended with fibers of larger diameter, such as wood pulp and/or cotton. This blend could be formed into a sheet of considerable thickness having very small inner spaces and interlocked layers. Commercial asbestos-cellulose sheets were usually 2.0 to 6.0 mm. thick; most sheets were standardized to a thickness in the 3.5-4.0 mm. range. The tortuous path between the openings provided capillaries of predictable average size and, therefore, predictable random entrapment of the solids to be removed. By varying the percentage of asbestos, the degree of fibrulization of the asbestos and the type of the cellulose, filter sheets could be produced with varying degrees of porosity. They could be as tight as 0.2 micron, which is approximately the tightness necessary for removal of all microorganisms.
These asbestos-cellulose sheets were dried and cut to an appropriate size for use on filter presses or cut into discs which could be fabricated into cartridges for insertion into cartridge housings. Because of the low cost of the components, these dry filter sheets of asbestos/cellulose were relatively inexpensive. They could be economically stored, transported and disposed of after use. As a result, until the 1970,s, these filter sheets of asbestos and cellulose were widely used throughout the world for presses and preformed cartridges. There are tens of thousands of presses and cartridge housings, which were built to hold such filter sheets and such cartridges, still in use.
For rough filtration or large volume filtration, loose media such as diatomaceous earth (skeletons of prehistoric diatoms) and/or perlite (a ground volcanic glass), have been used as depth filter media in pressure leaf filters. Often cellulose, asbestos or other fibers have been combined with such loose media or used as precoating materials to prevent migration of the filter aid particulates through the filter screen support.
For fine or ultimate filtration, porous membranes have also been used commercially. Although such membranes provide predictable, absolute screens against the passage of very small particles, they have very little solids holding capacity and tend to clog rapidly when challenged with any quantity of solids. Membranes are useful filter media for a final filtration step of a liquid from which the bulk of the solids has been removed by other processes but membranes are not efficient general filtration media.
Although its filtration abilities were excellent, asbestos became an undesirable filter medium. Various types of asbestos had been used over the years for many other purposes, ranging from brake linings to thermal insulation. In order to prevent fire at sea, which is a major danger to ships, asbestos insulation was used extensively in hundreds of ships built in the first half of the twentieth century. By the early 1970's the medical literature started reporting that shipyard workers were subject to various diseases which seemed related to contact with asbestos. More recently, other insulation workers who worked with asbestos have been reported as having similar health problems. The use or handling of asbestos in any form came to be considered dangerous. Asbestos became unacceptable to the general public.
Microfiberglass has been used as a substitute for asbestos in certain filter applications. However, the National Institute of Health has reported that a major cause of the health hazard of asbestos is related to the size and the nondegradable nature of the fibers and it is believed that microfiberglass could present a health hazard similar to that of asbestos.
As a result, there have been extensive efforts to develop a line of asbestos-free (and microfiberglass-free) filter sheets since the early 1970's. Both perlite and diatomaceous earth have been tried as substitutes for asbestos in the composition of filter sheets and they have now become widely used in combination with cellulose fibers in the fabrication of filter sheets.
Several drawbacks to filter sheets made with either diatomaceous earth or perlite have been noted. First, since these materials are particulate rather than fibrous they are incapable of bridging the space between cellulose fibers and cannot be interlocked into place as reliably as fibers can. In order to provide filtration sufficiently tight to retain small microbiological organisms, the media particles must be very small. Since these small particles are not firmly locked in place, they tend to migrate into the filtrate, contaminating the final product. This problem of media migration is increased whenever the relatively stiff, particulate-containing filter sheets are bent or scuffed. Even with the use of wet and dry strength resin binders, and even when the cellulose fibers are beaten to increase their holding capability, there can be no absolute guarantee against particles flowing to the filtrate.
Secondly, diatomaceous earth and perlite contain crystalline silicates. Crystalline silicates have been determined by the IARC Committee of the World Health Organization to be potential carcinogens. U.S. regulations require a health warning label on these crystalline silicate products. It is feared that the dangers of asbestos, or perceived dangers of asbestos, regarding problems of production, use and disposal could be applicable to diatomaceous earth and perlite, as well.
Finally, diatomaceous earth (and to a lesser extent, perlite), contain iron, aluminum and other metallic ions which can be leached into the filtrate. As little as 6 parts per million ferrous ion in beer can be detected by a trained taster; iron can also turn whiskey black and catalyze a protein-tannin precipitation in wine and alcohol-based cosmetics and drugs. Traces of copper can cause "a casse" (a cloud) in wine. In the pharmaceutical industry, filtration processes are believed to allow aluminum pick up in injectable drugs. Since reports in the medical literature connect aluminum uptake to certain diseases, metallic ion leaching is particularly undesirable in these circumstances.
The shortcomings of microfiberglass, diatomaceous earth and perlite as asbestos substitutes have made the search for an effective filtering medium without similar health risks more important. One such promising substitute has been cellulose esters made in fine fibrous form. The preparation and use of cellulose acetate filter material has been described in U.S. Pat. Nos. 3,842,007 and 3,961,007. Both these patents discuss the storage and use in a wet state of high surface area cellulose acetate, described as "rod-shaped fibers intermingled with fine fluffy filaments and having a surface area in the range of about 35 m.sup.2 /g and about 55 m.sup.2 /g". Such material was apparently intended for use in pressure leaf filters, which utilize filter beds formed in situ, at the point of use. U.S. Pat. No. 3,842,007 specifically noted that fluffy cellulose acetate could not be dried without destroying its usefulness as a filter material. Although U.S. Pat. No. 3,961,007 mentioned that cellulose acetate fibers could be advantageously mixed with other filter materials, such as cellulose fibers, the patent teaches that such other materials are to be added to the cellulose acetate solution prior to precipitation of the cellulose, acetate fibers. Further, such mixtures were to be 0.5:1 (331/3%) to 3:1 (75%) cellulose acetate: cellulose. The assumption that such a filtering material would be expensive is reinforced by the description in the patent of possible collection, washing and reuse of the material. No mention of forming preformed dry depth filter sheets is contained in these patents.
Other patents disclose other methods for making high surface area cellulose ester fibrils. U.S. Pat. No. 4,047,862 describes a method for making such material for an end use "as fillers in high opacity paper with high retention during the paper making process." U.S. Pat. Nos. 4,192,838 and 4,274,914 disclose yet another method for formation of the fibrils which were called "fibrets. " "The term `fibret` as employed herein identifies a high surface area, cellulose ester fibrillar material having a surface area in excess of 5.0 square meters per gram, lengths of less than 1,000 microns and diameters of about 0.5 to 50 microns." In each of these latter patents the material was described for use as a component in a very thin sheet-like material suitable for corrugating and use in the smoke filter of cigarettes, a product used in dry form which is 1/500 the thickness of a liquid depth filter sheet. Thus, cellulose ester fibrils, despite their reported capability since the mid-1970's as a filtration medium, have not been used in making a dry filter sheet which can be usable in a standard filter press or filter cartridge for liquid depth filtration. Cellulose ester material, which is relatively expensive compared to other filtration media, was not considered usable, in the quantities taught in the art, for disposable dry filter sheets.
Thus, since the 1970's, there has been a need for a filtering material which would be effective for filtering liquids to remove particles in the range of 0.2 to 30 microns and which would not be a potential health hazard at either the point of manufacture, the point of use, o the point of disposal. There has been a need for such an effective filtering material which could be made economically enough to be disposable. There has been a special need in the field for a filtering material which can be formed into dry filter sheets which can be cut to fit into the numerous existing filter presses and cartridge housings originally developed to use asbestos/cellulose or diatomaceous earth/cellulose filter sheets.
The present invention is a solution to these problems and provides a new sheet-like filtering material made only from biodegradable materials which have been "generally regarded as safe" by the U.S. Food & Drug Administration. This invention uses cellulose which, after it has been beaten, can be combined with relatively minor quantities of cellulose ester fibrils to form dry filter sheets which have effective filtering capabilities for fine filtration comparable to asbestos/cellulose filters but without the health hazards of the latter. The filtering material of this invention can be made into preformed filters and discs for cartridges to fit the existing filtration equipment and at an economic cost so that such sheets can be disposable. Leaching of particulates and health hazards are avoided with this invention.